Interpretive Summary: Although concentrations of the mycotoxin deoxynivalenol (DON) are usually correlated with kernel damage in wheat, there have been observations of unacceptably high DON in relatively plump, sound-appearing grain. We studied the effects of infections by the wheat scab fungus Gibberella zeae (asexual stage = Fusarium graminearum) that were later than the “normal” timing of mid-flowering. We also applied mist irrigation of varying durations to experimental plots. We found that in two of three years, infections 10 days after mid-flowering produced less kernel damage than infections at mid-flowering. However, DON concentrations were the same for the “late” infections as for the timely infections in two of three years. The normal period of maximum susceptibility to G. zeae appears to be close to 10 days in North Carolina winter wheat. In one of the three years, a high percentage of our samples (41%) had low kernel damage but levels of DON above the desirable ceiling (2 parts per million). We examined the effects of different infection timings and mist irrigation durations on this low-symptom, high-DON scenario, and found that the highest percentages of such samples occurred under conditions of late infection. Periods of rain soon after flowering probably favor the low-symptom, high-DON phenomenon.

Technical Abstract:
While deoxynivalenol (DON) concentrations in mature wheat grain are usually correlated with symptoms produced by Gibberella zeae infection, there have been numerous observations of unacceptably high DON in asymptomatic crops, which can lead to lower-than-expected milling reductions in DON. We conducted a field experiment with winter wheat to examine the effect of infection timing and post-anthesis moisture on grain quality and DON accumulation. Seven, eight, and seven soft red winter wheat cultivars were grown, respectively, in three successive years at a misted nursery in Kinston, North Carolina. Spikes were randomly selected for individual spray inoculation at 0, 10 or 20 days after anthesis (daa). Starting at anthesis, plots were subjected to either 0, 10, 20, or 30 days of mist. Inoculated spikes and non-inoculated controls were collected at harvest-ripeness, and the threshed grain was assayed for Fusarium-damaged kernels (FDK) and DON. In two of three years, percentages of FDK were significantly lower from 10-daa infections than from those at 0 daa, although DON concentrations were the same at the two inoculation timings in two of the three years. Our results indicate that the normal window of maximum susceptibility to head infections by G. zeae is close to or slightly less than 10 daa in North Carolina wheat. In two of three years, FDK-DON correlation was greater for 0- and 10-daa inoculations and for 0- to 20-daa misted treatments than for the later-inoculated or longer-misted treatments, respectively. The percentage of “low-FDK, high DON” (LFHD) observations (defined as FDK = 4.0%, DON = 2 µg/g) was much higher in 2007 than in 2005 or 2006 (41%, 14%, and 18%, respectively). In both 2006 and 2007, high percentages of LFHD observations (=60%) occurred under marginal disease conditions involving late infection. We conclude that late infection is an important factor leading to LFHD grain. Periods of rain soon after anthesis likely favor the low-symptom, high-DON scenario, and conditions that create greater within-crop variability of anthesis timing may also be important.